Advice to handle kid who cheats

Hello all, as title suggests I have a nephew who has recently taken to chess (been playing for lasy 12 months) and it has been fun to watch and help him with his learning and progress. He has slowly climbed his way to almost 1000 rating. However in last few daily games we have played he has pulled out 4 wins and a draw. Not close wins but dominating wins. I have checked analysis of the games and he is all of a sudden playing at +90% accuracy and in one 40 move game he did not even make one mistake or inaccurate move all game. For a 1000 rated player is that plausible?. We typically play on another website where my rating is just over 1900 so I am no slouch with chess but obviously still prone to making mistakes. When our last game got to maybe move 15 he had smothered me so much and had such an advantage i thought it was game over and after he did this crazy series of sacrifices i decided to check a chess engine and low and behold he followed the top line....in our current game I have been checking each of his moves against an engine and he is consistently picking top 1 or 2nd line. I then started to follow the chess engine line and he still proceeded to win but by the time i started following the engine it was already +4 so not much hope for me..so  I used a chess engine to prove my 1000 rated 12 year old nephew was also using an engine....but trying to convey this information to his father (my brother) was meet with flat out refusal and denial and some attacks against me because I was upset I lost to him etc  he did say he checked thr tablet he uses and he saw no evidence of using a chess engine. However when we play in person i destroy him easily so i am not sure how to proceed?  If i stop playing i will be blamed for being a sore loser but if i continue to play and he continues to (allegedly) cheat its not doing him anygood....

How do you encourage kids to play and discourage cheating without destroying relationships???

Any advice?

It is violation of the rule.

As mentioned, it is against chess.com's tos to discuss cheating in the public forums. 

You can discuss cheating in this club though: https://www.chess.com/club/cheating-forum

If you ever suspect someone of cheating, then just report them: https://support.chess.com/en/articles/8562517-how-do-i-report-someone